The overall objective of this proposal is to define the role of estrogen in the growth of 7,12-dimethylbenz(a)anthracene (DMBA)-induced rat mammary tumors: The following specific goals are set for the period proposed: 1) to improve my current technique on the in vitro incorporation of 3H-leucine by the tumor tissue so that the effect of hormonal stimulation can be demonstrated in a more sensitive manner. 2) to test whether the synergistic effect between estrogen and prolactin is exerted by the mechanism that, in the presence of one hormone, the tissue increases its sensitivity to another hormone by an increase in the amount of receptor for the latter. 3) to investigate whether the transformation of hormone-dependent tumors into autonomous tumors is due to an acquisition of tissue's capacity to synthesize estrogen. In this sense, an "autonomous" tumor may still be estrogen-responsive, despite the fact that it is able to survive in the absence of host's ovaries.